Biological control and removal of nitrogen from wastewater is based on four biological processes: (1) ammonification; (2) cell synthesis: (3) nitrification; and (4) denitrification. The interrelationship of these processes constitutes the nitrogen cycle in aqueous systems. A representation of the nitrogen cycle in wastewater treatment systems is depicted on Figure 12.1-1.
Ammonification is the biological conversion of organic nitrogen to the ammonium form. It can be expressed as:
Organic nitrogen + Microorganisms → NH3/NH+4 Eq. (12.1-1)
Nitrogen is an essential nutrient for microbial growth. Cell matter, generally described in simplified form as C5H7NO2, contains nominally 12 wt. % nitrogen. Waste activated sludge serves as a removal mechanism based on both bound nitrogen within the cell matter and also dissolved nitrogen compounds in water associated with the wasted sludge not recycled to the treatment plant.
Nitrification, as described in Eqs. 6.1-3 and 6.1-4, is the biological oxidation of ammonia. It occurs in two steps, first to the nitrite form, then to the nitrate form. Overall, 2 moles of oxygen are required to oxidize 1 mole ammonium ion, or 4.57 lb oxygen/lb N.
Denitrification, as described in Sections 6.1 and 11, is the process by which microorganisms convert nitrate or nitrite into nitrogen gas. The principal biochemical pathway for denitrification involves using oxygen bound up in the nitrate or nitrite, and not free or dissolved oxygen. A simplification of Eq. (6.1-5) can be expressed as:
4 NO + 2 H−3 2O → 2 N2 + 4 OH– + 5 O2 Eq. (12.1-2) Accompanying the formation of nitrogen gas, note that 1 mole of hydroxyl ions are produced for each mole of nitrate reduced.
Further, 1.25 mole of oxygen is produced for each mole of nitrate reduced, or 2.86 lb oxygen per lb N. Theoretically, 63% of the oxygen consumed in the nitrification reaction is produced during denitrification. This produced oxygen is available to supply the oxygen demand of organic substrate and microorganisms present in the anoxic reaction zone.
Nitrogen Forms
Nitrogen in aqueous treatment systems is present in a variety of forms. Petroleum and petrochemical raw wastewater will typically contain ammonia nitrogen, and depending on onsite manufacturing processes, organically bound nitrogen can also be present. During aerobic biological treatment, both ammonification of organic nitrogen and nitrification of ammonia nitrogen can occur, yielding oxidized nitrogen compounds. These oxidized species, predominantly nitrates, but some nitrites, can be reduced under anoxic conditions to yield nitrogen gas. A simplified representation of the nitrogen cycle within a wastewater treatment system is depicted in Figure 12.1-1.
Controlling nitrogen in effluents requires both an understanding of applicable specific discharge limits as they relate to nitrogen and analytical test methods and reported results. Discharge limits may be specified as one or more of the following: ammonia;
ammonia nitrogen; Total Kjeldahl Nitrogen; organic nitrogen; nitrates; nitrate nitrogen; nitrite; or nitrite nitrogen . Wastewater nitrogen measurement parameters, Standard Test Methods, and the relationship of these parameters are summarized in Table 12.1-1.
12.0 NITROGEN MANAGEMENT (Cont) Process Microbiology
Unlike nitrification, a broad range of bacteria can accomplish denitrification. Denitrifiers are commonplace in most natural environments. Denitrifiers can be readily sustained in aerobic systems because of their ability to use oxygen and efficiently oxidize organic matter. This is due in part to the fact that they are facultative: they can use either oxygen or nitrate as their terminal electron acceptor. The process where microbial cells generate energy involves transferring electrons from a reduced electron donor (e.g., an organic substrate) to an oxidized electron acceptor (e.g., oxygen, nitrate, or sulfate). Microbial metabolism ensures that the most efficient form of energy production is utilized. Thus, if oxygen is present, it will be used preferentially over nitrate. Likewise, if oxygen is not present, nitrate will be preferentially used over sulfate.
Sulfate reduction to sulfide and resulting odor production are not likely to occur within a treatment system that is anoxic (i.e., nitrate is present). Nitrate depletion through carbon overdosing, and thus leading to short term anaerobic conditions is also not likely to lead to sulfate reduction since the sulfate reducers will not have time to proliferate in numbers capable of significant sulfate reduction. Further, sulfate reducers are subject to poisoning in the aerobic zones of combined anoxic-aerobic treatment systems.
Process / Reactor Variations For Biological Denitrification
All biological processes for total nitrogen removal require that ammonification (of organic nitrogen) and nitrification precede actual denitrification. For denitrification to occur, nitrates must be present. Various reactor configurations, however, can be employed to achieve denitrification with and without recycle of the effluent from the nitrification reactor. For each configuration, suspended or attached growth systems can be utilized. ExxonMobil does not have an attached growth nitrification &
denitrification system in operation, but non-ExxonMobil refineries, literature, and pilot testing by EMRE indicate that attached growth can be considered for cost effective denitrification operations.
There are three primary process configurations for Nitrification / Denitrification. Descriptions of these primary configurations are as follows:
1. Separate Sludge Post-Denitrification (Figure 12.1-2a) - Separate sludge denitrification is a system where a clarifier is provided for each reactor with its own sludge recycle system. Multiple sludge or separate sludge systems operate at higher unit removal rates and consequently require lower reactor volumes. However, this advantage is normally overshadowed by several disadvantages relative to single sludge systems, including increased clarification requirements, inferior sludge properties, increased need for pH control, increased aeration requirements, and increased organic carbon requirements (as compared to pre-denitrification mode).
2. Single Sludge Post-Denitrification (Figure 12.1-2b) - In this configuration, the aerobic reactor is followed by the anoxic reactor and then by the post aeration reactor. Most of the organic carbon in the wastewater is consumed in the nitrification / organic removal stage. For denitrification to occur, an adequate amount of organic carbon must be present in the wastewater. Therefore, addition of an external carbon source is typically required.
3. Single Sludge Pre-Denitrification (Figure 12.1-2c) - In this single sludge configuration, the anoxic reactor precedes the aerobic reactor. A large portion (typically 4 to 6 times the influent rate) of the fully nitrified aerobic mixed liquor is recycled to the anoxic reactor to promote rapid denitrification. Depending on the desired total nitrogen removal, the organic material in the influent is used as a carbon source for the denitrifying bacteria. This configuration is applicable for refineries / chemical plants that have a fairly high influent TOC concentration. If the site does not have a sufficient amount of biodegradable TOC in the influent, an external carbon source such as methanol or sodium acetate must be added. Except for synthesis during denitrification in the anoxic zone, ammonia and organic nitrogen in the influent are untouched as they pass from anoxic to aerobic stage. The advantage of the pre-denitrification mode is that the wastewater organics can serve as the electron donor (carbon source) for the denitrification reaction. This type of denitrification system is found at the Slagen and Ingolstadt Refineries.
Alternative Reactor Designs
A variety of reactor designs and operating strategies have been applied to the nitrification / denitrification process. These are described below:
1. Concentric Reactors - This reactor within a reactor approach saves on both plot space and pumping of internal recycle, and is readily applied to a single sludge pre-denitrification system. The smaller inner circle volume acts as the anoxic zone, while the outer annulus volume serves as the aerobic nitrification reactor. Raw wastewater is fed to the anoxic zone where it is mixed with internal recycle from the aerobic zone. Effluent from the aerobic zone flows by gravity to a clarifier from where biomass is separated for return to the anoxic zone. This type of reactor is found at Slagen Refinery.
12.0 NITROGEN MANAGEMENT (Cont)
2. Cyclic Aeration - Alternating aerobic and anoxic zones can be achieved in a continuous-flow, activated sludge system by cycling the aerators on and off to create anoxic and aerobic zones. This approach, termed cyclical nitrogen removal (CNR) can be most effectively applied at existing treatment plants that must meet new or revised nitrogen discharge limits.
Modifications may be as minimal as installing baffles or timers to cycle the aeration equipment, but can also include providing internal recycle pumps and piping. If several alternating zones are used, raw wastewater may be step-fed to those downstream zones in which organic carbon or COD has been depleted and denitrification reaction rates have become carbon limited.
3. Oxidation Ditches or Racetrack Reactor - An oxidation ditch reactor uses looped trenches that provide a continuous circulation path for the wastewater. Aerators within the flow path supply both oxygen and motive force to the wastewater.
Conceptually, the oxidation ditch is an endless channel. Because only a portion of the mixed liquor is withdrawn each cycle, a high internal recycle ratio is achieved. Nitrification occurs in that portion of the loop immediately downstream of the aerators; oxygen deficient conditions prevail immediately upstream of the aerators, thereby providing anoxic conditions for the denitrification reaction. Oxidation ditch reactors are more typical in municipal service and have not been used in the ExxonMobil circuit.
4. Sequencing Batch Reactors - A variant of the fill-and-draw SBR described in Section 7.0 involves pulsing the aeration equipment in the activated sludge treatment unit on a timed cycle. This results in alternating aerobic and anoxic conditions being achieved on a temporal basis within the same reactor (as opposed to multiple reactors or zones in series). SBR denitrification systems have not been applied at ExxonMobil manufacturing sites.
Alternative Biological Processes
Biological processes less widely applied, or still in development, include facultative lagoons with algae harvesting and constructed wetlands. Facultative lagoons achieve nitrogen removal by both ammonia stripping and by algae synthesis.
Carbon dioxide produced from aerobic (surface layers) and anaerobic stabilization (bottom layers) is the carbon source for the algae, which photosynthetically produce biomass and oxygen. CO2 depletion associated with algae synthesis will result in an increase in pH; this can limit ammonia stripping. Significant ammonia stripping does not occur at a pH of greater than 8.5.
Surface constructed wetlands (characterized by a free water surface) rely on higher forms of plant life (relative to algae), e.g., duckweed, water hyacinths, to achieve nitrogen removal. This is effectively an attached growth system that relies on synthesis of new plant matter for nitrogen removal. Systems that rely on synthesis for nitrogen removal ultimately must plan for harvest and disposal of the resultant biomass.
12.2 DESIGN CONSIDERATIONS
Similar to conventional and nitrifying activated sludge systems, the design of an anoxic denitrification process must consider the following: 1) compliance with regulatory effluent requirements, 2) feed wastewater characteristics, variability, and pretreatment requirements, 3) selection of process configuration, 4) selection of reactor type, and 5) need for pilot plant data.
The design considerations for all these items are discussed in Section 6.2 of this Design Practice. In addition, there are specific considerations that apply for the denitrification process, which are discussed below.
Effluent Nitrogen Limits
Discharge requirements may be specified or expressed as one or more of the following: ammonia; ammonia-nitrogen; Total Kjeldahl Nitrogen; organic nitrogen; nitrates; nitrate-nitrogen; nitrite; or nitrite-nitrogen. Wastewater nitrogen measurement parameters, Standard Test Methods, and the relationship of these parameters are summarized in Table 12.1-1.
Actual design must address the different forms of nitrogen present in the effluent, and their impact on meeting specified limits.
For example, if only ammonia or ammonia and nitrite (due to toxicity) are limited, then only nitrification is needed. If nitrate is limited, or a combination of ammonia, nitrite and nitrate, then nitrification alone is not sufficient and denitrification is also needed. Although not typical, if organic nitrogen (TKN) is limited then one must also take into account volatile suspended solids (microbial matter) in the effluent. Cell matter, generally described in simplified form as C5H7NO2, contains nominally 12 wt. % nitrogen. At 20 mg TSS/L and a VSS/TSS ratio of 0.8, the organic nitrogen in the suspended cell matter will contribute 2 wppm nitrogen in the effluent.
pH
Denitrification is a net producer of alkalinity. The theoretical alkalinity production is 3.57 mg of CaCO3 per mg of nitrogen reduced. See Table 12.2-1. The production of alkalinity will raise system pH and offset some of the loss of alkalinity associated with nitrification in combined systems. Nitrification consumes 7.15 mg of alkalinity (expressed as CaCO3) per mg of ammonia-nitrogen nitrified. The pH in the anoxic reactor should be kept between 6.5 and 8.5. The specific optimum will vary
12.0 NITROGEN MANAGEMENT (Cont) Temperature
The operating temperature for the denitrification process is approximately the same as in the nitrification process. The maximum allowed temperature is approximately 99°F (37°C) due to sensitivity of Nitrobacter. The optimum temperature for the nitrification / denitrification system is approximately 86°F (30°C). Temperatures below 68°F (20°C) will substantially decrease the rate of substrate removal such that nitrification may not occur at all, resulting in no denitrification.
Dissolved Oxygen (D.O.)
D.O. level is very critical for both nitrifying and denitrifying reactors since it directly controls the growth rate of the nitrifiers. For a combined nitrification and denitrification process, the D.O. in the aerobic reactor should be established just above 2 mg/L and a D.O. of 0 mg/L should be targeted for the anoxic reactor. Care should be taken not to allow the D.O. to rise much above 2 mg/L in the aerobic reactor because it tends to carry over to the anoxic reactor. This D.O. will preferentially be consumed over that available from the nitrate.
Dissolved oxygen will inhibit the activity of denitrifying enzymes. A dissolved oxygen level of ≥ 0.2 mg/L has been reported to inhibit the initiation of denitrification. Once initiated, the rate of denitrification will be significantly reduced (relative to that at zero D.O.) if the system dissolved oxygen level is increased. Relative denitrification rates (relative to those at zero D.O) of 50 % at 0.2 mg/L and 10 % at 2 mg/L have been reported. (Reference- Randall, page 51)
Mixed Liquor Recycle Rate and Recycle Ratio
Effluent nitrogen levels are related to the mixed liquor recycle in single sludge pre-denitrification systems. Typical mixed liquor recycle ratios of 4-6 are common; higher ratios of 10-20 may be required to achieve low residual nitrogen in the effluent.
Energy consumption and pumping costs can be significant. Plant layout should take into consideration such energy costs and look to minimize distances and hydraulic gradients between the anoxic and aerobic zones.
Assuming complete denitrification of the NO3––N recycled to the anoxic stage and neglecting nitrogen assimilation, the required recycle ratio (internal mixed liquor + return sludge) is given by Eq. 12.2-1 (Reference 5)
e 3
e 4 o
4
N) (NO
N) (NH N)
R (NH
−
−
−
= +− − + – 1 Eq. (12.2-1)
where: R = Recycle ratio (multiples of raw influent wastewater) (NH –N)+4 o = Influent ammonium-nitrogen, mg/L
(NH –N)+4 e = Effluent ammonium-nitrogen, mg/L (NO –N)−3 e = Effluent nitrate-nitrogen, mg/L
If partial denitrification is required, R = % denitrification/(100-% Denitrification). The following demonstrates the impact of increased denitrification targets on recycle rates:
PERCENT DENITRIFICATION REQUIRED TO ACHIEVE
EFFLUENT LIMIT
MINIMUM RECYCLE RATIO
70 2.3
80 4
85 5.7
90 9
95 19
Based on practical considerations, and a desire to limit pumping requirements, maximum design denitrification targets are typically in the 80 – 85 % range. Higher percent denitrification can be achieved, but at significantly higher capital and operating cost.
12.0 NITROGEN MANAGEMENT (Cont) Power Input to Anoxic Zone
The purpose of mixing within the anoxic zone is simply to keep the mixed liquor solids suspended while minimizing surface turbulence, and thus minimize transfer of oxygen from the atmosphere. Accordingly, significantly less power input (HP/Mgal) is needed than for aerobic zones as shown in Table 12.2-2. As noted above, mixed liquor recycle flow is substantially greater than the raw wastewater influent rate. The energy required, however, to properly mix this recycle flow with the influent is likely to be greater than that required for solids suspension. In that case, it is recommended that these streams be mixed immediately upon entering the anoxic zone by adding them in close proximity to each other..
Organic Substrate to Nitrogen Ratio
Carbonaceous matter in the wastewater is used as the energy source and electron donor for the denitrification reaction. When there is an insufficient amount of carbon in the denitrification system, denitrification can be inhibited. Should there be a lack of carbonaceous matter in the system, a readily biodegradable external carbon source such as methanol, acetic acid, sodium acetate, or certain raw wastewaters can be used. Factors to consider when choosing an external substrate include cost, low sludge yield, and toxicity / handling issues.
The quantity of organic substrate required is dependent on the specific organic selected. Postulated denitrification oxidation-reduction reactions for common external carbon sources are given in the table below:
SUBSTRATE DENITRIFICATION REACTION EQUATION NO.
Methanol 5 Eq. (12.2-2)CH3OH + 6NO3− →3N2 + 8H2O+1CO=3 + 4HCO−3
Acetic Acid 5 Eq. (12.2-3)CH3COOH+ 8NO−3 → 4N2 + 6H2O+ 2CO2 + 8HCO−3 Sodium Acetate 5 Eq. (12.2-4)CH3COO− + 8NO−3 → 4N2 + 4H2O+ 3CO=3 + 7HCO−3
Based upon these reactions, minimum theoretical substrate requirements can be determined based upon the oxygen demand exerted by the specific organic substrate employed. See Table 12.2-3. These requirements are the minimum needed to consume free oxygen produced from denitrification, and should be increased by a nominal factor of 1.3 - 1.5 to assure substrate requirements for nitrogen reduction and cell synthesis are satisfied. One must add to this value any additional substrate needed to deplete dissolved oxygen present and to account for the reduction of any nitrite-nitrogen present. For example, the denitrification equation using methanol and including cell synthesis is as follows:
−3
NO + 1.08 CH3OH + H+ → 0.065 C5H7O2N + 0.47 N2 + 0.76 CO2 + 2.44 H2O Eq. (12.2-5) The following empirically derived equation shows the amount of methanol required where nitrate, nitrite and dissolved oxygen are present:
Using the theoretical values in Table 12.2-3, (excluding nitrite) the equation would be as follows:
÷÷øö
where: Cm = Required methanol amount, grams (NO3–N)o= Initial Nitrate-N amount, grams (NO2–N)o= Initial Nitrite-N amount, grams
DO = Initial dissolved oxygen amount, grams
Since most refinery applications will use carbon in the feed wastewater as part or all of the carbon source for denitrification, the carbon to nitrate-nitrogen ratio is needed for refinery wastewaters to determine if sufficient carbon is available. Because the source of carbon in all refinery wastewaters is different, bench scale testing is needed to be sure of the carbon to nitrate ratio needed. Bench testing for Ingolstadt Refinery indicated a carbon to nitrate ratio of at least 2 was needed. For that specific
12.0 NITROGEN MANAGEMENT (Cont) Solids Residence Time (SRT)
SRT is one of the most critical design parameters of the nitrification / denitrification system. For a single sludge combined nitrification / denitrification system, the SRT is defined as:
e)
The microbial species responsible for nitrification have relatively low specific growth rates and only grow in the aerobic zone.
Since nitrification must precede any denitrification reaction, it is necessary to maintain a SRT sufficiently large to insure a suitable nitrifier population. The recommended design SRT for a nitrification / denitrification system is 25 days.
Hydraulic Retention Time (HRT)
The hydraulic retention time is the length of time the raw influent wastewater is retained in the BIOX. Typically, the HRT for the aerobic reactor should be long enough to obtain the desirable SRT. Per Section 6.3, Standard Procedures - Design Conditions, at least 18 hours is recommended for nitrifying systems (aerobic zone). The anoxic zone in an activated sludge nitrification-denitrification system should have an HRT of at least 4 hours. The anoxic volume should not be more than 20 to 33
% of the total reactor volume. Acceptability of the design should be demonstrated by pilot testing. Shorter HRT's may be acceptable.
Actual detention time (ADT) within the anoxic and aerobic reactors is a fraction of the HRT and is a strong function of the internal recycle and sludge recycle. It is the quotient obtained by dividing reactor volume by the actual total volumetric flowrate to the reactor. A minimum actual detention time of one hour is recommended for the anoxic zone; lower values should only be considered for retrofit applications where existing reactor volume is fixed.
Actual detention time (ADT) within the anoxic and aerobic reactors is a fraction of the HRT and is a strong function of the internal recycle and sludge recycle. It is the quotient obtained by dividing reactor volume by the actual total volumetric flowrate to the reactor. A minimum actual detention time of one hour is recommended for the anoxic zone; lower values should only be considered for retrofit applications where existing reactor volume is fixed.